The present invention generally relates to channel quality reporting, and particularly relates to channel quality reporting in a multi-carrier wireless communication environment.
Currently, the 3rd Generation Partnership Project (3GPP) is initiating work on evolving the Wideband CDMA standard (WCDMA). This evolution aims at improving packet data transmission performance, e.g., through higher throughput, increased data rates, reduced delays, and support for flexible spectrum allocation. Of particular interest is higher bandwidth support for packet data services.
One candidate for this evolution is a multi-carrier WCDMA (MC-WCDMA) based air-interface. A MC-WCDMA air interface protocol provides multiple WCDMA frequency carriers operating in parallel. As an example, MC-WCDMA could offer up to four parallel frequency carriers in the downlink direction, each carrier having a chip rate of 3.84 Mcps, corresponding to approximately 5 MHz bandwidth. A similar approach is taken for cdma2000, where a multi-carrier mode is specified. Normally, cdma2000 uses a single carrier, known as cdma2000 1x. However, cdma2000 provides a multi-carrier mode denoted as cdma2000 3x. Three frequency carriers are used in parallel according to cmda2000 3x.
An MC-WCDMA compatible terminal is capable of receiving multiple frequency carriers, while a conventional WCDMA terminal is only capable of receiving a single carrier and is not aware of other carriers. This ensures that MC-WCDMA is backward compatible with conventional WCDMA.
WCDMA also has been enhanced with High-Speed Downlink Packet Access (HSDPA) for efficient downlink packet-data support (Rel-5 of the WCDMA specification). Based on HSDPA, the WCDMA specifications identify a new downlink transport channel, the high-speed downlink shared channel (HS-DSCH). HS-DSCH is based on a shared channel transmission scheme. A certain amount of channelization codes and transmission power in a cell can be considered a common resource that is dynamically shared among users primarily in the time domain when a shared channel transmission scheme such as HS-DSCH is deployed. Shared-channel transmission makes more efficient use of available code resources in WCDMA. For example, the shared code resource onto which the HS-DSCH is mapped consists of up to 15 codes with fixed spreading factor 16. The HS-DSCH also supports new features such as fast link adaptation, fast hybrid ARQ with soft combining, and fast channel-dependent scheduling that rely on, and are tightly coupled to, the rapid adaptation of transmission parameters to instantaneous radio conditions.
Fast link adaptation enables the HS-DSCH to rely on rate adjustment while transmission power is kept constant instead of compensating for significantly and rapidly varying downlink radio conditions by power control. This is commonly known as rate adaptation, rate control, or link adaptation and is more efficient than power control for services tolerating short-term variations in the data rate. Furthermore, the HS-DSCH may use spectral-efficient 16 or 64 QAM modulation when channel conditions permit in order to further increase the capacity and data rates.
Fast hybrid ARQ with soft combining enables a mobile terminal to rapidly request retransmission of erroneous data, which leads to substantial delay reduction and also higher capacity compared to release 99 of WCDMA. By using soft combining, the mobile terminal combines information from both the original transmission and any subsequent retransmissions prior to a decoding attempt. This further increases the capacity and adds robustness against link adaptation errors.
Fast channel-dependent scheduling allows a scheduler to control to which users the shared channel transmission should be directed at a given time instant. Channel-dependent scheduling implies that the instantaneous radio-channel conditions are taken into account during the scheduling process, which significantly increases capacity and resource utilization. Short-term variations in the radio conditions are exploited by transmitting to a user which experiences favorable instantaneous channel conditions, e.g., a fading peak, while a certain degree of fairness is still maintained over the long term. To fulfill the requirements on short delays and rapid adaptation set by the techniques discussed above, the corresponding functionality is typically located in the Node B of a radio access network.
HSDPA also provides a new downlink control channel for WCDMA, the High-Speed Shared Control Channel (HS-SCCH), which is code multiplexed with the HS-DSCH. HS-SCCH carries control information necessary for the demodulation of the HS-DSCH by a mobile terminal. The control information, which is required for each 2 ms WCDMA Transmission Time Interval (TTI), includes the identity of the currently scheduled terminal, hybrid ARQ related information, and HS-DSCH transport format parameters selected by the link adaptation mechanism. The HS-DSCH is shared mainly in the time domain but can also be shared in the code domain. Time domain scheduling makes the most use of the rapid adaptations possible with the HSDPA feature, but when data transmitted to one mobile terminal does not completely fill the HS-DSCH channel, several mobile terminals may be scheduled at the same time using different subsets of the available codes (code domain sharing). Since only the currently scheduled terminal needs to receive the HS-SCCH, there is typically only one, or a few HS-SCCHs configured in each cell. However, HS-DSCH capable terminals are required to monitor up to four HS-SCCHs.
The HS-DSCH and HS-SCCH use staggered timing. The staggered timing structure reduces mobile terminal complexity since it allows the terminal to receive parts of the control information and to use the information to configure its HS-DSCH receiver before beginning to receive the HS-DSCH transmission. However, a more complicated power allocation algorithm is needed in the base station when the HS-DSCH and HS-SCCH are transmitted using staggered timing. With staggering, subsequent HS-DSCH transmission activity must be taken into account when the HS-SCCH power is set. This may require a static HS-SCCH power reservation, leading to reduced system capacity. Staggering also introduces an additional and seemingly unnecessary delay for user data.
HSDPA further provides a new uplink control channel for WCDMA, the High Speed Dedicated Control Physical Channel (HS-DPCCH), which is provided to each terminal using HS-DSCH services. The HS-DPCCH is used by the hybrid ARQ mechanism in the mobile terminal to request retransmissions of erroneously received transport blocks. The HS-DPCCH is also used to transmit reports of the instantaneous downlink channel quality observed by the mobile terminal to the corresponding Radio Base Station (RBS). Instantaneous channel quality conditions are conventionally measured by devices such as cellular phones and are broadly referred to as Channel Quality Indicators (CQIs). CQI values may correspond to Signal-to-Noise Ratio (SNR), Signal-to-Interference+Noise Ratio (SINR), received signal power or strength level, supportable data rates, supportable modulation and/or coding rates, supportable throughput, etc.
CQI information is conventionally transmitted from a wireless receiver such as a cellular phone to a corresponding transmitter such as an RBS via physical layer signaling. In one example, CQI information is transmitted over the HS-DPCCH in compliance with the HSDPA protocol for WCDMA-based systems. Wireless transmission systems use CQI information to assist in radio resource allocation. For example, CQI information may be used to determine transmission scheduling among multiple receivers, select suitable transmission schemes (e.g., the number of transmit antennas to activate), determine bandwidth allocation, select spreading codes, determine modulation and coding rates, etc.
CQI information is conventionally transmitted in the form of a CQI message. Receivers form CQI messages by first measuring channel quality, e.g., SNR or SINR. The receiver then accesses a standardized CQI table where the table contains ranges of uniquely indexed CQI values. The range in which the measured channel quality falls is identified by selecting the corresponding index value. The selected index value is mapped to a sequence of channel quality information bits, e.g., using a (20,5) block coding technique where 5 bits are encoded into a 20 bit-codeword for error protection. The encoded channel quality information bits are then mapped onto the HS-DPCCH and transmitted as a message.
The HSDPA features described above provide channel quality feedback for a single downlink carrier in a MC-WCDMA environment. That is, the current HS-DPCCH physical channel structure only supports feedback of one CQI value per 2 ms interval, corresponding to one downlink carrier. However, channel quality feedback is desired for all carriers in a MC-WCDMA system to take full advantage of the advanced radio communication features offered by HSDPA.